It is known that the wheels of a vehicle are, generally, made up of a cylindrical metal rim having, at the axial extremities, annular raised edges between which is defined a channel for slotting in an elastic tyre, the side portions of which, so-called “beads”, are fitted fast up and against the annular raised edges themselves.
The need is also known to perform frequent balancing operations which consist in the fitting of weights, made of lead or other material, in correspondence to predetermined points of the wheel and along the rim and the need to check the aptitude of the wheel to correctly spin following a geometric analysis of the rim and of the tyre.
In fact, during wheel rotation, the fitting of the weights offsets any irregularities of the tyre and/or the rim which would lead to the production of vibrations or stresses during vehicle movement.
To perform such operations, balancing machines are commonly used comprising a supporting structure for the wheel gripping and rotation means, of the type of a spindle axially rotatable by means of motor means, onto which the wheel rim is keyed by means of suitable engagement and centring parts.
The measurement of wheel unbalance is determined during rotation by suitable electronic or electro-mechanical devices, such as force transducers fitted along the spindle.
To the measurement of unbalance are generally added other characteristic measurements, such as the measurement of the roundness of the wheel, of the eccentricity of the wheel, the amount of wear of the tread, the conformation of the rim or other wheel irregularities, normally performed by means of suitable measurement sensors with or without contact (e.g., feelers or optical sensors).
Once the necessary measurements have been taken, the machine is able to calculate the size and position of the balancing weights to be fitted to the wheel rim to offset the wheel irregularities.
The fitting of the balancing weights is normally done manually by an operator in one of more precise points of the wheel rim indicated by the machine.
The no-contact measurement of the dimensional characteristics of the wheel is commonly done by means of systems which use the triangulation method and which, generally speaking, have a moving mechanical support that moves a punctiform laser beam emitter and a sensor device, of the type, e.g., of a linear CCD (Charged-Coupled Device), suitable for determining the laser light reflected by the wheel.
In particular, by positioning the system close to the wheel rotation axis and moving the mobile support to perform a series of successive measurements, the inner dimensions of the rim and the position of the balancing planes for calculating the balancing weights to be fitted can be determined.
Such known systems allow obtaining, in a relatively economic way, the same information obtained by means of a traditional contact sensor, of the type, e.g., of a mechanical feeler or the like.
These known systems, however, are not without drawbacks.
In particular, the movement of the mechanical support, needed to perform a series of successive measurements, requires considerable time and slows down the entire process of acquisition of the dimensional characteristics of the wheel.
The document U.S. Pat. No. 5,054,918 describes an appliance that can be used to detect the shape and the dimensions of a wheel fitted to a balancing machine.
Such appliance has a laser blade emitter suitable for projecting a laser line on the portion of wheel to be measured.
This way, the laser blade plane produced intercepts the wheel profile, projecting a broken or curved laser line onto the surface of the wheel itself.
The appliance also has a light-sensitive sensor device, of the type of a camera or the like, arranged in a predefined position with respect to the laser emitter and suitable for determining the laser line projected on the wheel.
Appliance processing means are suitable for determining the structural and dimensional characteristics of the wheel, starting from the shape of the projected laser line.
A similar appliance is also illustrated in the patent document US 2006/0042380. These appliances nevertheless are also susceptible to upgrading, aimed in particular at allowing a more precise determination of the structural and dimensional characteristics of the wheel and/or a faster determination process.
In this respect, the fact is underlined that to carry out a correct measurement of the unbalance of a wheel, this must be made to rotate at a rather high speed.
The cameras fitted on traditional balancing machines however have the capacity to detect images with a very limited sampling frequency and are practically unusable when the wheel is made to rotate at high speed.
The traditional balancing machines equipped with a viewing system, therefore, usually contemplate an initial stage of rotation of the wheel at low speed, for the acquisition of the images, and a subsequent stage of rotation of the wheel at high speed, for the detection of the unbalance.
This results is an inconvenient extension of the times of execution and it is for this reason that the balancing machines with viewing systems are also very slow.